1. Field of the Invention
The present invention relates to a data receiver and a method of determining its threshold value. More specifically, the invention relates to a data receiver used in a wireless local area network (LAN) according to the standard IEEE (Institute of Electrical and Electronics Engineers) 802.11.
The invention also specifically relates to a method of determining the threshold value of a data receiver in which method he threshold value based on the received signal strength indication (RSSI) is adaptively changed so as to select and set an optimum threshold value in noisy environment.
2. Description of the Background Art
Conventional modems are adapted to be out of operation in order to reduce the power consumption thereof when a received signal is estimated by means of an RSSI signal. A wireless digital synchronized system disclosed in Japanese patent laid-open publication No. 2003-273796 has a configuration in which in order to reduce power consumption in a waiting period of time in which no data are transmitted and received between master and slave stations the power supply to the transmitter of the master station is interrupted by the first control while no data are transmitted and received, and when the RSSI voltage is detected higher than a predetermined level or predetermined demodulated data are detected the first control determines a call originating to switch the transmitter on accordingly.
A received signal is estimated by means of comparison of an RSSI signal obtained from the received signal with a predetermined threshold value, i.e. RSSI threshold value. In a highly noisy environment, however, the RSSI value of a received signal increases so high that the estimation of the received signal becomes difficult.
For example, in an intermittent receiving method disclosed in Japanese patent laid-open publication No. 2001-94505, an existing noise is erroneously determined as a carrier signal existing so that control frequently shifts itself to a continuous receiving state, therefore, power being supplied to the receiver circuit in a longer period of time. As a result, there is a phenomenon of short battery life. The mechanism of the phenomenon is understood such that, when the threshold values for determining a detected voltage are set so that a carrier is detected in the first stage of intermittent receiving and no carrier is detected in the second stage of intermittent receiving, a higher-noise level condition in the received frequency band is detected in such a manner that there is detected a carrier in the first stage of intermittent receiving although there are detected neither data nor carrier in the second stage of intermittent receiving to thereby render itself back to the first stage, the processes being repeated between the first and second stages of intermittent receiving. As the operating time increases in the second stage of intermittent receiving with the intermittent operation period shorter, the battery life becomes shorter.
In the intermittent receiving method taught in publication No. 2001-94505, the receiving control circuit is provided with a counter function which counts the number of repetitions between the first and second stages of intermittent receiving, and if the number of repetitions exceeds a predetermined value, the threshold value is then increased for determining whether or not the career exists in the first stage of intermittent receiving. By means of this processing, transfer to the second stage of intermittent receiving becomes more difficult and thus it is possible to keep the battery life long in noisy environment.
The threshold value adjusted by the above-mentioned conventional method can converge to a constant value when the environmental condition for radio wave propagation is calm. However, the environmental condition for radio wave propagation is not always calm, so that it may be disturbed turbulently. This turbulence includes noise deterioration and/or amelioration. The threshold value taught in the intermittent receiving method of publication No. 2001-94505 is only increased corresponding to the increase of the noise level. The intermittent receiving method of publication No. 2001-94505 is only directed to the threshold value responding in the deteriorating direction so that it cannot follow the change very well. That may cause unnecessary actions to be increased for adjusting to obtain the optimum threshold value, thus increasing power consumption.